1. Field of the Invention
The present invention relates to a process for preparing an unsaturated dicaroboxylic acid diester, and particularly it relates to a process for preparing efficiently an unsaturated dicarboxylic acid diester from a conjugated diene by use of a specific catalyst.
2. Description of Related Arts
Generally, an unsaturated dicarboxylic acid diester such as 2-butene-1,4-dicarboxylic acid diester or the like is important as an intermediate material of a variety of chemical products such as a precusor of adipic acid or the like.
A method for carbonylating a conjugated diene in the presence of an alcohol, carbon monoxide and oxygen is hitherto known, and in order to increase the produced amount of an unsaturated dicarboxylic acid diester a reaction has been carried out substantially in an anhydrous state by the use of a dehydrating agent. However, the conventional methods with use of a dehydrating agent have the following problems (1) - (3) unsolved and needed to be improved.
That is, (1) though an organic compound is used as the dehydrating agent, it becomes a quite different compound after the reaction and is difficult to be reused as a dehydrating agent; (2) the reaction becomes complicated due to by-products derived from the dehydrating agent and the selectivity to carbonylation is decreased; and (3) the operation for realizing an anhydrous state in the reaction system or for the separation after the reaction is complicated.
Thus, the conventional method with use of a dehydrating agent is not suitable for practical use in industry. There has also been proposed a method of adding an aliphatic or alicyclic primary or secondary amine as a method for obtaining the abovementioned objective product (Japanese Patent Application Laid-Open No. 48942/1982). However, this method was also insufficient as a method for industrially preparing an unsaturated dicarboxylic acid diester.
As described above it is the state of things that no industrially effective method has hitherto been known for producing an unsaturated dicarboxylic acid diester such as 2butene-1,4-dicarboxylic acid diester or the like without any dehydrating agent.
The present inventors have earnestly conducted research for the purpose of solving the aforementioned problems in the conventional methods and developing a method for efficiently producing an unsaturated dicarboxylic acid diester from an conjugated diene.
As a result thereof, it has been found that the purpose can be accomplished by the use of a catalyst comprising (a) a platinum metal, a salt thereof or a complex compound of a platinum metal, (b) a salt of a metal selected from copper, iron and manganese and (c) an alkali metal compound, an alkaline earth metal compound, rare earth metal compound, a metal compound of the groups IIIA, IVA, IVB, VA, VB or VIA or a metal carbonyl complex. The present invention has thus been completed based on the knowledge.